(CNN) — Health experts are doubling down on pleas for all eligible American to get vaccinated — and boosted — against Covid-19 as the Omicron and Delta variants spread.

Less than two weeks after Omicron was first identified by South African scientists, it’s already become the dominant variant in that country.

It’s too early to tell whether Omicron is more contagious than Delta, which is still the dominant variant in the US.

But the Delta variant is already fueling another increase in Covid-19 hospitalizations in some states as the weather gets colder and more people gather indoors.

Despite several unknowns about the Omicron strain, doctors say booster shots are the best weapon against Omicron, Delta and even more variants down the road. Here’s why:

Boosters have worked well against other new variants

Moderna, Pfizer/BioNTech and Johnson & Johnson are testing the efficacy of their vaccines against the Omicron strain, and it might be two weeks before the results are known.

But Dr. Anthony Fauci said there’s plenty of reason to get vaccinated and boosted as soon possible.

“We know from experience that even with variants that are not specifically directed at by the vaccine, such as the Delta variant, if you get the level of antibody high enough, the protection spills over to those other variants,” said Fauci, director of the National Institute of Allergy and Infectious Diseases.

The Omicron variant has not yet been detected in the US. But if it’s not already stateside, it’s arrival is almost inevitable, said Dr. Francis Collins, director of the US National Institutes of Health.

“We do believe that this new variant, which will probably come to our shores, will also be something vaccines and boosters can help you with,” Collins said.

Of course, the Omicron is different from other variants. “It has a lot of mutations — more than 50,” Collins said. “That’s a new record.”

More than 30 of those mutations are in the spike protein — the part of the virus targeted by leading Covid-19 vaccines.

Scientists are trying to learn how much Omicron might evade the antibodies produced from vaccination or natural infection.

“We worry if the spike protein is of a different shape, maybe the antibodies won’t stick quite as well. That’s the reason for the concern,” Collins said.

But here’s the good news: “All of the previous variants, which have also had differences in the spike protein, have responded to vaccines — and especially boosters,” Collins said.

The Delta variant keeps spreading this holiday season

The battle against the Delta variant is far from over as some states grapple with increasing hospitalizations.

At least 20 states have reported more patients hospitalized with Covid-19 this past week than the previous week, according to data Tuesday from the Department of Health and Human Services.

Many of those states are in colder parts of the country, such as the Northeast and Midwest.

“The winter surge may be here, or we’re just at the beginning,” said Gov. Kathy Hochul of New York, where new hospitalizations are increasing.

Doctors are dreading another winter surge as the weather gets colder and more people gather indoors.

But unlike the last holiday season, Covid-19 misery this season is largely preventable.

“Your best protection against Delta is to get vaccinated, and if you’ve already been vaccinated and six months have passed since you got Pfizer or Moderna, get your booster; two months since J&J, get your booster,” Collins said.

“That was a reason already. But now add Omicron to the mix.”

Vaccines (and boosters) help prevent more variants

Anyone sick of hearing about new variants — or the possibility of new Covid-19 restrictions — can help stop the cycle by getting vaccinated and boosted, experts say.

“The virus mutates when people get infected. It doesn’t mutate in the air,” said Dr. Jorge E. Rodriguez, an internal medicine specialist based in Los Angeles.

“There is no such thing as a good infection, even if you survived it with minimal symptoms,” he said.

“Even though you’ve got infected and you did fine, guess what? You may very well have contributed to mutations that will be stronger.”

But coronavirus still has plenty of opportunity to infect unvaccinated Americans and vaccinated Americans who need a booster shot.

About 40.7% of Americans aren’t fully vaccinated, according to data Monday from the US Centers for Disease Control and Prevention.

Among the 59.3% who are fully vaccinated, 20.5% of them have gotten a booster dose.

Get your booster, CDC urges

While the CDC doubled down on its recommendation for eligible Americans to get booster shots, more Americans may be eligible for boosters soon.

“Everyone ages 18 and older should get a booster shot either when they are six months after their initial Pfizer or Moderna series or two months after their initial J&J vaccine,” CDC Director Dr. Rochelle Walensky said Monday.

“The recent emergence of the Omicron variant (B.1.1.529) further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against COVID-19,” Walensky said in a written statement.

“I strongly encourage the 47 million adults who are not yet vaccinated to get vaccinated as soon as possible and to vaccinate the children and teens in their families as well because strong immunity will likely prevent serious illness.”

Teens ages 16 and 17 might soon be eligible or boosters as well.

Pfizer is expected to seek authorization from the US Food and Drug Administration for its vaccine booster for 16- and 17-year-olds, a source familiar with the plan told CNN.

Collins said the US needs everyone to help end the pandemic. And vaccinations and boosters are “the best chance we’ve got to drive this Covid-19 pandemic away.”

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